1. Field of the Invention
The present invention is directed to a drive system for a vehicle comprising an internal combustion engine and a torsional vibration damping arrangement wherein the internal combustion engine is switchable between operating modes of different performance capability.
2. Related Art
A drive system, shown schematically in FIG. 1, comprises an internal combustion engine 5 which delivers an unsteady torque owing to the periodic combustion process. Accordingly, a disturbance torque with fixed orders is superposed on the nominal torque. These orders depend on the combustion process (two-cycle or four-cycle process) and on the number of cylinders. Thus the main exciting order of a four-cylinder four-cycle engine is the second order, that of a three-cylinder four-cycle engine is the 1.5th order, and that of a four-cylinder two-cycle engine is the second order.
The system has a control device 2, which detects the load condition and rotational speed at the engine based on signals, e.g., vehicle speed, from the engine or from the rest of the drivetrain 3 (gearbox, differential, axle), whereupon it chooses whether all of the cylinders or only some of the cylinders are to be operated or which combustion process is to be used in the engine; that is, generally speaking, it chooses between operating modes of different performance capability.
In order to stabilize the drivetrain, vibration reducing systems 4, which are configured, e.g., based on the excitation order are used as torsional vibration damping arrangements. For this purpose, it was possible heretofore to proceed from fixed orders which were not variable during operation.
For the purpose of reducing consumption and therefore also emissions, it is possible to change the quantity of cylinders or the combustion process, i.e., the operating mode, during operation. An internal combustion engine always has its optimal efficiency at a relatively high load so that the specific fuel consumption is lowest at that time. The common partial load states in real operation consume too much fuel in relation to the demanded power.
One solution to this problem is to switch off individual cylinders 1 and operate the rest of the cylinders at a higher load level and, therefore, at a higher level of efficiency. This switching off can be carried out, for example, by turning off the fuel injection for half of the cylinders and/or changing the control times via a variable valve control.
As an alternative solution to switching off cylinders, a four-cycle engine can be optimally configured for partial load operation and can be switched to two-cycle operation at peak load when needed.
Particularly in the cycle relevant for consumption, only loads which are very small in relation to the full engine are demanded; thus in the NEDC (New European Driving Cycle) the engine with an installed power of up to 200 kW only puts out approximately 10 kW because the required accelerations are very moderate. Also, for routine operation of a vehicle the full load is requested only in rare cases so that switching off cylinders or switching operating modes offers a great savings potential.
When the excitation order changes, the distance with respect to rotational speed or frequency between the operating range and the natural frequency of the vibration reducing system also changes and, therefore, so does the quality of vibration reduction.